(a) Field of the Invention
The present invention relates to processes for producing polyamide-imide resins and, more particularly, it relates to economical processes for producing high molecular weight polyamide-imide resins exhibiting excellent heat resistance and melt flowability.
(b) Description of the Related Art
As methods of producing polyamide-amides, solution polymerization method with aromatic diisocyanates (see Japanese Patent Application Publication No. 48-19274), precipitation polymerization method with aromatic diisocyanates (see Japanese Patent Application Publication No. 54-44719), a solution polymerization method with pyromellitic acid chlorides (see Japanese Patent Application Publication No. 42-15637), and a solution polymerization method with aromatic diamines (Japanese Patent Application Publication No. 49-4077) are known. In case of a solution polymerization method with aromatic diisocyanates, undesirable side reactions tend to occur from the start of polymerization, causing a difficulty in production of linear polymers having high molecular weight. The polymers produced by the method are therefore too poor in melt flowability to be suitable for the use of molding materials or the like.
Precipitation polymerization method with aromatic diisocyanates has problems in safety in working environment and cost since reaction is carried out using highly poisonous nitro compounds or expensive sulfolane type solvents. Further, the method, due to it's difficulty in controlling molecular weight, has difficulty in quality control of the product. Furthermore, the method tends to induce side reactions like the above-described solution polymerization method with aromatic diisocyanates, and therefore the produced polymers are also inferior in melt flowability. A solution polymerization method with pyromellitic acid chlorides is disadvantageous in cost since it needs a step for purifying the by-produced halogen compounds. Further, in case of producing modified polymers, the method has such a disadvantage as the restriction in materials to be used.
On the other hand, a solution polymerization method with aromatic diamines is free from these problems and is a useful method well-balanced in cost and melt flowability and heat resistance of the product polymers. The method is practically carried out using a dehydration catalyst, for example, phosphoric acid type catalysts such as phosphoric acid and polyphosphoric acid, boric acid type catalysts such as boric acid and boric anhydride, and phosphorous triesters such as triphenyl phosphite. It is known that catalytic effects of these catalysts substantially differ according to the kind thereof.
That is, while a single catalyst system of phosphoric acid, polyphosphoric acid or boric acid can exhibit sufficient catalytic effect with a small amount, it however needs a long time reaction at a high temperature of 200.degree. C. or more. Therefore, even if N-methylpyrrolidone (boiling point: 202.degree. C.), which is a high boiling point solvent, is used as the solvent for synthesis, there will frequently occur undesirable phenomenon that the generating resins having high molecular weight and high viscosity are baked or stuck to the surface of reactor wall in the course of the high temperature and long time synthetic process. While a single catalyst system of phosphorous triesters can achieve high molecular weight polymerization in a reaction at a relatively low temperature of 190.degree. C. or less, it is required in an amount equivalent to the amount of the condensation-reactive groups of the acid component or amine component, and the use of a large quantity of expensive catalyst causes problems of cost and difficulty in controlling molecular weight.